It is known that conventional mobile radio telephone systems are of cellular configuration; i.e., the respective mobile radio telephone system is formed by a multiplicity of radio cells and a base station is allocated to each radio cell. If a mobile station (for example, a mobile telephone), is located in the geographic area of a radio cell, communication with the mobile station within the mobile radio telephone system takes place between the mobile station and the base station allocated to the current cell. If the mobile station is moving from one cell into another cell, the communication link existing between the mobile station and the base station allocated to the “old” cell must be handed over to the base station allocated to the “new” cell. This process is called “handover”.
So that such a handover can be performed as rapidly and as free of interference as possible, it is necessary for the mobile station located in the geographic area of a cell to continuously monitor and evaluate the signals transmitted by the base stations of adjacent cells in order to obtain system information of the adjacent cells which can be used for setting up communication between the base station of the new cell and the mobile station as rapidly as possible when entering into an adjacent new cell.
This monitoring of adjacent cells by the mobile station is made more difficult due to the fact that the base stations in the individual cells use different scrambling codes. For the mobile station located in a cell, this means that it must be continuously adjusted for decoding and evaluating the transmit signals from base stations of adjacent cells in accordance with the different-scrambling codes. The more adjacent cells there are, the longer the monitoring of adjacent cells will take.
At national boundaries which, as a rule, also represent separating lines between two different mobile radio telephone network operators, the case may occur that the two network operators of different nationalities use the same frequency band. If a mobile station of one network operator is close enough to the border of the other network operator, the mobile station can receive the transmit signal of the cell of the other network operator on the other side of the border. In UMTS (Universal Mobile Telecommunication System) systems, for example, the signals received by the mobile station are unambiguously allocated to a cell or to a network operator by reading a system information item which is transmitted via the so-called broadcast control channel (BCCH) of the base station of the respective cell.
Reading the system information item of an adjacent cell is a time-consuming process so that, for example, at a national boundary, the case may occur that there is only one adjacent cell of the same operator for a cell in which the mobile station is currently located whereas there are several adjacent cells of the other network operator so that the system information items of the adjoining cells of the other network operator must be read and evaluated during the monitoring of adjacent cells. In other words, the mobile station needs a large amount of time for identifying the signal received from a network operator. Cumulatively, this time can become very large if there is a very large number of adjacent cells of a foreign network operator transmitting at the same frequency as one's own network operator; i.e., if a very large number of adjacent cells of a foreign network operator are visible at the same frequency for the mobile station.
To illustrate, a dashed national boundary is shown in FIG. 5 which separates a cellular mobile radio telephone network of a network operator A and a cellular mobile radio telephone network of a network operator B. According to FIG. 5, the mobile radio telephone network A includes individual cells A0–A6 whereas the mobile radio telephone network B includes individual cells B0–B6. If, for example, a mobile station of network operator A is located in cell A6, the mobile station can monitor both the adjacent cells A3 and A4 of its own mobile radio telephone network A and the adjacent cells B0, B3, B4 and B1 of the foreign mobile radio telephone network B. That is, the mobile station reads both system information of cells A3 and A4 and system information of cells B0, B3, B4 and B1 even though this would actually only be required for cells A3 and A4 since cells B0, B3, B4 and B1 are cells of a foreign mobile radio telephone network.
The present invention is directed toward providing a method for monitoring adjacent cells in a mobile radio telephone system, and a correspondingly designed mobile radio telephone system, via which the time needed for monitoring adjacent cells can be reduced.